This invention relates to titanium based alloys, and more particularly to titanium niobium aluminide alloys containing a substantial portion of an orthorhombic crystal form comprising at least 25 percent of the volume fraction of the alloy microstructure.
New titanium aluminide alloys herein referred to as titanium niobium aluminides, and disclosed in U.S. Pat. No. 5,032,357, incorporated herein by reference, are comprised of about 18 to 32 atomic percent aluminum, 18 to 34 atomic percent niobium, and the balance essentially titanium. The titanium niobium aluminide alloys were found to have improved high temperature strength, and comparable or improved fracture toughness as compared to prior art trititanium aluminide alloys. It is believed an orthorhombic phase comprising at least 25 percent of the volume fraction of the microstructure of the titanium niobium aluminide alloys provides an important contribution to the improved properties of the alloys. Alloy compositions are sometimes hereafter shown as, for example, Ti-25A1-25Nb for an alloy comprised of 25 atomic percent aluminum, 25 atomic percent niobium, and the balance titanium.
A variety of elements have been added to the known trititanium aluminides such as molybdenum, tantalum, vanadium, hafnium, and silicon. For example, see Blackburn, M. J. and W. P. Smith, "Research to Conduct an Exploratory Experimental Investigation of Alloys" Report No. AFWAL-TR-80-4175, U.S. Air Force, WRDC, November, 1980, and U.S. Pat. Nos. 4,788,035, and 4,292,077. Some of the mechanical properties from the prior art titanium aluminides in the above references is shown in Table 1 below to compare the affect of vanadium in the alloys.
TABLE I ______________________________________ Mechanical Properties of Prior Art Titanium Aluminide Alloys Composition 0.2% Ultimate (Atomic Percent) Y.S. Tensile Strength Elongation Al Nb V Ti (ksi) (ksi) (percent) ______________________________________ 25 14 Bal 120 142 2.1 25 14 Bal 116 134 2.6 25 10 4 Bal 105 127 1.7 25 10 4 Bal 105 128 2.0 22.5 10 Bal 50.06 64.2 16.5 22.5 10 Bal 70.04 110.2 20 22.5 5 5 Bal 108.2 138.4 6 24 11 Bal 4 24 9 2 Bal 2.3 ______________________________________
From the tensile testing properties shown above in Table 1, it can be seen that the addition of vanadium in such prior art trititanium aluminides has had the result of reducing the room temperature ductility of the alloys. Although it has been found useful to replace niobium with vanadium in some trititanium aluminide alloys to reduce the alloy density, ductility has been reduced.
It is an object of this invention to provide titanium niobium aluminide alloys comprised of vanadium, and having improved room temperature ductility.
It is another object of this invention to provide titanium niobium aluminide alloys comprised of vanadium, and having improved room temperature fracture toughness.